How To Use Winston to Log Node.js Applications

Introduction

An effective logging solution is crucial to the success of any application. In this guide we'll focus on a logging package called Winston, an extremely versatile logging library and the most popular logging solution available for Node.js applications, based on NPM download statistics. Winston's features include support for multiple storage options and log levels, log queries, and even a built-in profiler. This tutorial will show you how to use Winston to log a Node/Express application that we'll create as part of this process. We'll also look at how we can combine Winston with another popular HTTP request middleware logger for Node.js called Morgan to consolidate HTTP request data logs with other information.

After completing this tutorial you will have an Ubuntu server running a small Node/Express application. You will also have Winston implemented to log errors and messages to a file and the console.

With these prerequisites in place, we can build our application and install Winston.

Step 1 — Creating a Basic Node/Express App

A common use for Winston is logging events from web applications built with Node.js. In order to fully demonstrate how to incorporate Winston we will create a simple Node.js web application using the Express framework. To help us get a basic web application running we will use express-generator, a command-line tool for getting a Node/Express web application running quickly. Because we installed the Node Package Manager as part of our prerequisites, we will be able to use the npm command to install express-generator. We will also use the -g flag, which installs the package globally so it can used as a command line tool outside of an existing Node project/module. Install the package with the following command:

sudo npm install express-generator -g

With express-generator installed, we can create our app using the express command, followed by the name of the directory we want to use for our project. This will create our application with everything we need to get started:

express myApp

Next, install Nodemon, which will automatically reload the application whenever we make any changes. A Node.js application needs to be restarted any time changes are made to the source code in order for those changes to take effect. Nodemon will automatically watch for changes and restart the application for us. And since we want to be able to use nodemon as a command-line tool we will install it with the -g flag:

sudo npm install nodemon -g

To finish setting up the application, change to the application directory and install dependencies as follows:

cd myApp

npm install

By default, applications created with express-generator run on port 3000, so we need to make sure that port is not blocked by the firewall. To open port 3000, run the following command:

sudo ufw allow 3000

We now have everything we need to start our web application. To do so, run the following command:

nodemon bin/www

This starts the application running on port 3000. We can test that it's working by going to http://your_server_ip:3000 in a web browser. You should see something like this:

It's a good idea at this point to start a second SSH session to your server to use for the remainder of this tutorial, leaving the web application we just started running in the original session. For the rest of this article, we'll refer to the SSH session we've been using so far and that is currently running the application as Session A. We will use the new SSH session for running commands and editing files, and we'll refer to this session as Session B. Unless otherwise noted, all remaining commands should be run in Session B.

Step 2 - Customizing the Node.js Application

The default application created by express-generator does a great job at getting us started, and it even includes the Morgan HTTP logging middleware we'll be using to log data about all HTTP requests. And since Morgan supports output streams, it makes a nice pairing with the stream support built into Winston, enabling us to consolidate HTTP request data logs with anything else we choose to log with Winston.

By default, the express-generator boilerplate uses the variable logger when referencing the morgan package. Since we will be using morgan and winston, which are both logging packages, it can be confusing to call either one of them logger. So let's change that by editing the app.js file in the root of the project and making some changes.

To open app.js for editing, use the nano command:

nano ~/myApp/app.js

Find the following line near the top of the file:

~/myApp/app.js

...
var logger = require('morgan');
...

Change it to the following:

~/myApp/app.js

...
var morgan = require('morgan');
...

We also need to find where the variable logger was referenced in the file and change it to morgan. While we are at it, let's change the log format used by the morgan package to combined, which is the standard Apache log format and will include useful information in the logs such as remote IP address and the user-agent HTTP request header.

To do so, find the following line:

~/myApp/app.js

...
app.use(logger('dev'));
...

Change it to the following:

~/myApp/app.js

...
app.use(morgan('combined'));
...

These changes will help us better understand which logging package we are referencing at any given time after we integrate our Winston configuration.

Exit and save the file by typing CTRL-X, then Y, and then ENTER.

Now that our app is set up we are ready to start working with Winston.

Step 3 — Installing and Configuring Winston

We are now ready to install and configure Winston. In this step we will explore some of the configuration options that are available as part of the winston package and create a logger that will log information to a file and the console.

To install winston run the following command:

cd ~/myApp

npm install winston

It's often useful to keep any type of support or utility configuration files for our applications in a special directory, so let's create a config folder that will contain the winston configuration:

mkdir ~/myApp/config

Now let's create the file that will contain our winston configuration, which we'll call winston.js:

touch ~/myApp/config/winston.js

Next, create a folder that will contain your log files:

mkdir ~/myApp/logs

Finally, let's install app-root-path, a package that is useful when specifying paths in Node.js. This package is not directly related to Winston, but helps immensely when specifying paths to files in Node.js code. We'll use it to specify the location of the Winston log files from the root of the project and avoid ugly relative path syntax:

npm install app-root-path --save

Everything we need to configure how we want to handle our logging is in place, so we can move on to defining our configuration settings. Begin by opening ~/myApp/config/winston.js for editing:

With these variables in place, we can define the configuration settings for our transports. Transports are a concept introduced by Winston that refer to the storage/output mechanisms used for the logs. Winston comes with three core transports - console, file, and HTTP. We will be focusing on the console and file transports for this tutorial: the console transport will log information to the console, and the file transport will log information to a specified file. Each transport definition can contain its own configuration settings such as file size, log levels, and log format. Here is a quick summary of the settings we'll be using for each transport:

level - Level of messages to log.

filename - The file to be used to write log data to.

handleExceptions - Catch and log unhandled exceptions.

json - Records log data in JSON format.

maxsize - Max size of log file, in bytes, before a new file will be created.

maxFiles - Limit the number of files created when the size of the logfile is exceeded.

colorize - Colorize the output. This can be helpful when looking at console logs.

Logging levels indicate message priority and are denoted by an integer. Winston uses npm logging levels that are prioritized from 0 to 5 (highest to lowest):

0: error

1: warn

2: info

3: verbose

4: debug

5: silly

When specifying a logging level for a particular transport, anything at that level or higher will be logged. For example, by specifying a level of info, anything at level error, warn, or info will be logged. Log levels are specified when calling the logger, meaning we can do the following to record an error: logger.error('test error message').

We can define the configuration settings for the file and console transports in the winston configuration as follows:

By default, morgan outputs to the console only, so let's define a stream function that will be able to get morgan-generated output into the winston log files. We will use the info level so the output will be picked up by both transports (file and console):

We now have our logger configured, but our application is still not aware of it or how to use it. We will now integrate the logger with the application.

Step 4 — Integrating Winston With Our Application

To get our logger working with the application we need to make express aware of it. We already saw in Step 2 that our express configuration is located in app.js, so let's import our logger into this file. Open the file for editing by running:

nano ~/myApp/app.js

Import winston near the top of the file with the other require statements:

~/myApp/app.js

...
var winston = require('./config/winston');
...

The first place we'll actually use winston is with morgan. We will use the stream option, and set it to the stream interface we created as part of the winston configuration. To do so, find the following line:

~/myApp/app.js

...
app.use(morgan('combined'));
...

Change it to this:

~/myApp/app.js

...
app.use(morgan('combined', { stream: winston.stream }));
...

Exit and save the file.

We're ready to see some log data! If you reload the page in the web browser, your should see something similar to the following in the console of SSH Session A:

There are two log entries here - the first for the request to the HTML page, the second for the accompanied stylesheet. Since each transport is configured to handle info level log data, we should also see similar information in the file transport located at ~/myApp/logs/app.log. The output in the file transport, however, should be written as a JSON object since we specified json: true in the file transport configuration. You can learn more about JSON in our introduction to JSON tutorial. To view the contents of the log file, run the following command:

So far our logger is only recording HTTP requests and related data. This is very important information to have in our logs, but how do we record custom log messages? There will certainly be times that we want this ability for things such as recording errors or profiling database query performance, for example. To illustrate how we can do this, let's call the logger from the error handler route.

The express-generator package includes a 404 and 500 error handler route by default, so we'll work with that. Open the ~/myApp/app.js file:

This is the final error handling route that will ultimately send an error response back to the client. Since all server-side errors will be run through this route, this is a good place to include the winston logger.

Because we are now dealing with errors, we want to use the error log level. Again, both transports are configured to log error level messages so we should see the output in the console and file logs. We can include anything we want in the log so be sure to include some useful information like:

err.status - The HTTP error status code. If one is not already present, default to 500.

To test this, let's try to access a page in our project that doesn't exist, which will throw a 404 error. Back in your web browser, attempt to load the following URL: http://your_server_ip:3000/foo. The application is already set up to respond to such an error, thanks to the boilerplate created by express-generator. Your browser should display an error message that looks like this (your error message may be more detailed than what is shown):

Now take another look at the console in SSH Session A. There should be a log entry for the error, and thanks to the colorize setting it should be easy to find.

As for the file logger, running the tail command again should show us the new log records:

tail ~/myApp/logs/app.log

You will see a message like the following:

{"level":"error","message":"404 - Not Found - /foo - GET - ::ffff:72.80.124.207","timestamp":"2018-03-07T17:40:10.622Z"}

The error message includes all the data we specifically instructed winston to log as part of the error handler, including the error status (404 - Not Found), the requested URL (localhost/foo), the request method (GET), the IP address making the request, and the timestamp of when the request was made.

Conclusion

In this tutorial, you built a simple Node.js web application and integrated a Winston logging solution that will function as an effective tool to provide insight into the performance of the application. You can do a lot more to build robust logging solutions for your applications, particularly as your needs become more complex. We recommend that you take the time to look at some of these other documents: